It is possible to convert renewable energy such as wind, wave, tidal energy or water current flows into electrical energy by using a turbine assembly to drive the rotor of a generator, either directly or by means of a gearbox. Other renewable-energy devices can be used to convert solar energy into electrical energy.
Renewable-energy devices can be connected together in clusters. Separate clusters of renewable-energy devices can then be connected together to a point of common coupling or collection point, this connection typically being made by means of a transformer with protective switchgear. The power output from the collection point is then transmitted to its destination (e.g. in the case of an off-shore wind turbine farm then this might be an onshore converter station which provides the power to a supply network or power grid) after being transformed and optionally rectified to a suitably high transmission voltage.
In the case where a renewable-energy device (e.g. a wind turbine or subsea turbine) includes a generator then it will typically also include a circuit breaker to interrupt fault currents. The circuit breaker can include off load isolators with safety earthing provision to allow a faulty generator to be bypassed during maintenance or repair.
Such arrangements are characterised by having interconnecting cables that incur significant charging current and eddy current losses as a result of high voltage ac (HVAC) operation. When a low impedance fault occurs within a particular cluster then it is accepted that power output from the cluster is interrupted until protecting switchgear interrupts the fault as a result of the collapse in line voltage whilst high magnitudes of fault current flow. The fault current is initially limited only by the combined impedances of all the power sources that are electrically connected to the cluster but the associated circuit breaker will then operate after a delay of up to about 150 ms. Cabling and protective switchgear must therefore be rated to withstand the thermal and mechanical effects of a significant current overload. The fault current must also be interrupted by the circuit breaker which is then exposed to rated line voltage after current interruption. The risk of re-strike in protective switchgear is significant and large surge arrestors can be required. In large, high power systems, the magnitude of the current overload may only be limited by appropriate choice of passive impedances, thereby adding further to the cost, complexity, size and reactive voltage drop of the power collection and transmission system. In such ac systems it is commonly a requirement that all generators contribute a substantial reactive current when line voltage drops below a particular threshold as part of a grid fault ride through strategy and, although generators incorporate actively controlled power electronics, they are not permitted to substantially reduce the above fault current magnitude.
Some arrangements use high voltage dc (HVDC) transmission, particularly for offshore wind or subsea turbine farms. Arrangements that use HVDC transmission do not have the charging current and eddy current losses that are inherent to HVAC systems. They also provide the flexibility to optimise the transmission voltage (i.e. the voltage carried by the transmission cable) and current choice without having to consider the charging current constraints. The power converter that is used to interface the transmission cable to the supply network or power grid can actively contribute to grid stability and quality of power supply. However, HVDC transmission also suffers from the disadvantage that it is necessary to actively rectify the ac power output of the generators in a converter station which sometimes needs to be located offshore, e.g. on a suitable platform.
Hybrid circuit breakers have been proposed for HVDC arrangements that employ mechanical contacts that open after current in the contact system has been reduced to zero, or even reversed, by dedicated and active current interruption means. These active current interruption means are typically complex and the term ‘hybridisation’ is used to describe the integration of mechanical switch contact functionality with the functionality of the dedicated and active means of current interruption. The active means typically includes power electronic switches, their switching aid networks (snubbers) and non-linear surge arresters. It is commonplace for the use of such hybrid circuit breakers to be proposed as a means of interrupting dc fault currents that flow in high power static power converter equipment and thus severe cost, efficiency and size penalties are incurred as a result of a requirement to employ two sets of power electronic equipment. A number of hybridisation techniques have been proposed where vacuum switchgear contacts start to open and a resonant commutation circuit is connected across the contacts, thereby causing periodic and short lived current reversals to occur in the low pressure metal vapour arc that forms between the progressively opening vacuum switch contacts. The arc is permitted to extinguish because cathode spot activity at low current density is sporadic and particularly rapid. However, the risk of re-strike has not been reliably addressed in such systems since the components of the resonant commutation circuit are very large if the resonant frequency is reduced to a sufficiently low value to permit the arcing system a conservatively large recovery time, and there is a practical motivation to reduce or minimise the size of the circuit components. The limitation of such hybridisation techniques is in their reliance upon a high frequency ac commutation mode whilst available vacuum switch components are typically optimised for line operation at 50 or 60 Hz for commercial reasons.
Conventional vacuum switches (sometimes called vacuum circuit breakers) are used in HVAC protection systems and the periodic current reversals that inherently occur in such systems have a fundamental bearing on the operation of these devices. The basic function of the vacuum switch is to interrupt fault current. The vacuum switch can be reset (either automatically or manually) to resume normal operation. The vacuum switch includes contacts that can be opened and closed by a mechanical actuation system. The mechanical actuation system can be triggered in response to the presence of a fault current or a manual command and the opening of the contacts starts asynchronously with respect to the ac line voltage and current. Since ac power factor may be anywhere between 0 pf lag to 0 pf lead, the ac current waveform can be phase shifted within a range of one half cycle of the line frequency relative to the ac line voltage waveform. The contacts open at a high acceleration rate and the ability of the gap between contacts to withstand voltage after current has been interrupted increases correspondingly, typically attaining the rated performance in about 7 ms after first contact separation. However the opening of the contacts whilst current is flowing generally does not cause interruption of the current until the next zero crossing in the ac waveform occurs. In a first example, the contacts may start to open at the start of a half cycle of current. This half cycle of current will then flow between the contacts as a low pressure metal vapour arc (or vacuum arc) without interruption until a short time after the next reversal of current, this time being defined by the chopping behaviour of the vacuum switch. In this example the current interruption occurs after the contacts have been fully separated since the typical 7 ms contact opening phase is shorter than the typical 8 or 10 ms half cycle duration. Immediately after the current interruption the vacuum switch will experience a transient recovery voltage between the contacts that is defined by the sum of the ac line voltage at that instant and a resonant transient. The ac line voltage at that instant depends on power factor and it is possible that peak line voltage is experienced. The resonant transient is caused by the response of the connected ac network to the chopping behaviour of the vacuum switch. In this example, the contact gap is fully open and has its maximum voltage withstand capability at the time of current interruption and consequent generation of a transient recovery voltage. In a second example, the contacts may start to open a small fraction of 1 ms before the current reversal when the gap between contacts may be a small fraction of their fully open gap. Under these circumstances three distinct behaviour types may be experienced: (i) the current may not be interrupted and the switching operation will continue as for the first example, (ii) the current may be interrupted but the contact gap may be insufficient to withstand the transient recovery voltage and the arc between contacts may re-strike, thereafter the switching operation will continue as for the first example, and (iii) the current may be interrupted whilst the contact gap is sufficient to withstand the transient recovery voltage but the switch may be susceptible to re-strike. Moreover, in a development of behaviour type (ii)—which may be referred to as type (iv)—after re-striking the vacuum switch may subsequently interrupt current and re-strike repetitively at a repetition frequency that is defined by the response of the cable interconnection system in conjunction with the stray impedances in the vacuum switch circuit. When the asynchronous nature of vacuum switch actuation and the power factor of the circuit whose current is interrupted are such as to promote a risk of type (iv) behaviour then the use of particularly large surge arrestors may be required. The susceptibility of a power system to type (iv) behaviour is strongly dependent upon the chopping characteristics of the vacuum switch.
The term ‘chopping’ is used to describe the extremely rapid decay of current that is forced by the extinction of a low pressure metal vapour arc between the contacts of the vacuum switch. Once chopping is completed, typically <100 ns after initiation, current no longer flows between the contacts of the vacuum switch unless re-strike occurs. The transient recovery voltage mentioned above and its rate of application must be limited to less than the time-variable and increasing dielectric withstand between the opening or opened contacts in order to prevent re-strikes. The chopping behaviour of conventional vacuum switches is extremely complex but occurs when current is below a particular threshold for a particular length of time. This threshold and its time dependency are subject to contact conditions that were prevailing before the contacts were opened. It is also important to note that arc extinction/re-ignition behaviour is cyclic, sporadic and can be characterised in terms of having a cathode spot lifetime that varies according to a statistical distribution. In over-current faults and ac load breaking circumstances, the chopping occurs at an instantaneous current that increases with the current that flows in the preceding half cycle. Since the inductively stored energy that is associated with the magnitude of the chopping current has an influence on both extinction and the likelihood of re-ignition, it is not surprising that the efforts of vacuum switch designers have focused on chopping current reduction.